The HZPT model replaces the traditional halo model two-halo term (eqn. REF ) with the Zel'dovich Approximation (ZA), or the leading-order Lagrangian perturbation theory (LPT) power spectrumUsing an alternative perturbative substitute for the two-halo term can also prove fruitful, and we provide an example in Section REF , but by default in this paper we stick with the ZA. [1]}, [2]}, [3]}, [4]}.
The ZA provides a beyond-linear-theory description of large scales, including large-scale nonlinear bulk flows.
A benefit of the ZA is that the IR resummation that would be necessary in Eulerian perturbation theory (SPT) or in an EFT extension thereof is not required at several-percent accuracy, [5]}, [6]}, [7]}, as the Baryonic Acoustic Oscillation (BAO) wiggles are already captured quite well by the ZA [8]}.
More importantly for the purpose of this paper,
ZA provides a useful ansatz for halo model
extension, since a compensated halo profile added to
ZA provides a description which is consistent
with perturbation theory in the regime of
its validity on large scales (Section II of SV15).
To address the known deficit in power in the ZA power spectrum on large scales, SV15 matched the ZA power spectrum to that of SPT to write the amplitude of the one-halo term that also describes small scales, which we now address.
